Wind asymmetries such as wind shear, upflow and yaw misalignment produce asymmetric loads on the wind turbine blades that involve, on one side, extreme loads and, on the other side, fatigue that can lead to damages in the blades and other wind turbine components.
For reducing said damaging effects, the prior art teaches the use of an individual pitch control added to the collective pitch control and also yaw control. The pitch and/or yaw commands for reducing said asymmetric loads are calculated using measurements of said loads or of displacements caused by them.
An example of this prior art can be found in US 2006/002792 where a method for reducing loads and providing yaw alignment in a wind turbine includes measuring displacements or moments resulting from asymmetric loads on the wind turbine. These measured displacements or moments are used to determine a pitch for each rotor blade to reduce or counteract asymmetric rotor loading and a favourable yaw orientation to reduce pitch activity. Yaw alignment of the wind turbine is adjusted in accordance with the favourable yaw orientation and the pitch of each rotor blade is adjusted in accordance with the determined pitch to reduce or counteract asymmetric rotor loading. As pitch control is relatively faster than yaw control it is generally more actively used for asymmetric load reduction than yaw alignment.
Such an individual pitch control derived from measurements of the effects of asymmetric loads has however the following drawbacks, particularly when it shall be installed in wind turbines already in operation:                a large increase in pitch actuation, implying a lower life in the pitch bearings;        it may need modifications in some wind turbine subsystems (hydraulic group and refrigeration);        the installation of the apparatus needed for measuring loads and/or displacements implies a high costs.        
The present invention focuses on finding a solution for these drawbacks.